1. Field of the Invention
This invention generally relates to medical instruments and more specifically to surgical retrieval baskets for removing an object from a body, particularly calculi that can form in the biliary and urinary systems, and a method for making such surgical retrieval baskets.
2. Description of Related Art
Recently developed medical instruments reduce the invasiveness and potential trauma previously associated with various medical procedures. The removal of calculi in the form of kidney stones, gallstones and the like from the body is one area where this effort is meeting with success. Various instruments now permit the removal of kidney stones and gallstones without the need for major surgery.
Some of these instruments incorporate miniaturized grasping forceps. Others incorporate a mechanical retrieval basket as described in the following U.S. Pat. Nos.
4,590,938 (1986) Segura et al
4,611,594 (1986) Grayhack et al
4,625,726 (1986) Duthoy
4,807,626 (1989) McGirr
5,064,428 (1991) Cope et al.
Ser. No. 08/258,121 (1994) Bates et al.
The Segura et al. patent discloses a surgical extraction device that can be inserted through the working channel of an endoscope for removing stones and the like from the kidneys or the ureter or biliary duct. The device includes a retrieval basket of relatively large diameter that is extendable from the distal end of a sheath and collapsible when withdrawn into the sheath. Outwardly bowed flat wire spring strips form the retrieval basket. These strips extend generally axially of the sheath and join at respective distal and proximal ends. The retrieval basket has a generally bulbous form that is relatively stiff due to the spring strip construction and that facilitates dislodgment and capture of stones.
The Grayhack et al. patent discloses another retrieval basket that is useful during the removal and/or destruction of calculi. A smooth outer tubular sheath encases a stranded helical wire cable terminating in a protective tip at the working or distal end of the device. When the cable is extended distally, the spring wire strands at the working end of the cable expand to form a retrieval basket. The distal end of this device additionally includes an expandable distal portion for protecting surrounding tissue during withdrawal of the device and calculi.
The Duthoy patent discloses an extraction device that includes a retrieval basket formed from a plurality of wires spaced about and outwardly from an imaginary extension of the center line of a hollow cable. A filiform extends distally from the distal end of the retrieval basket to extend past a stone and to allow the basket to be threaded around and onto the calculi.
The McGirr patent discloses an extractor included a self-closing retrieval basket at the distal end of a catheter with a flexible control line for opening the basket from the proximal end of the catheter. The basket assumes a normal position wherein it is in a compact closed form. Pulling on the control line flexes the strips to open the basket. When the control line is released, the strips relax and surround the calculi or object being removed.
The Cope et al. patent discloses a stone retrieval basket having superelastic individual wire loops secured together at the apex of the loops to define a distal end of a basket which can be insert through and beyond a distal end of an elongated tube. Sleeves which secure the wires in a spaced relation defines the proximal end of the basket so as to define a basket having a bulbous shape.
The Bates et al. patent application, that is a co-pending application commonly assigned with this application, discloses a surgical retrieval basket comprising axially extending wires that are grouped in sets of filaments. The wires are normally maintained in an overlying sheath in a compact condition and form an enlarged basket upon retraction of the sheath. The filaments of a set are relatively closely angularly spaced with the sets being relatively widely spaced to provide a greater number of contact points with entrapped calculi without significantly increasing the manipulations necessary to capture such calculi in the basket.
These and other surgical extractors using retrieval baskets have certain common characteristics. Each retrieval basket comprises a plurality of strands in the form of individual helical or flat wires substantially equiangularly spaced about the retrieval basket which is collapsible into a compact form. In some retrieval baskets the strands are formed along substantially straight lines when the basket is in the compact form (i.e., so-called "flat wire baskets"); in others, the individual strands extend along a generally helical path (i.e., so-called "helical wire baskets). Each instrument generally include a plurality of three or more strands, with the flat wire basket comprising an even number due to manufacturing constraints, as explained below.
Forming such wire baskets generally comprises grouping a plurality of axially extending wires having shape memory properties, such as stainless steel, to extend along parallel axes. Securing to wires together defines a basket subassembly having a distal and proximal end between which each of the wires extend. An operator uses a forming fixture, generally hand operated, and cold-works the wires of the subassembly to define the shape and form of the basket as either a helical wire or straight wire basket.
To form a straight wire type basket the forming fixture generally receives and secures distal and proximal ends on a straight wire axis. Generally, the forming fixture includes a mount fixing one of the ends along the axis and another mount biased so that the other end can move along the axis toward the one end during the forming process. A forming tool, generally comprising a linearly extending member corresponding to the desired radial extension of the basket to be formed, engages an opposed pair of the wires mediate the ends of the wires to urge the wires radially outward from the wire axis defined between the ends of the subassembly. Specifically, the tool rotates (e.g., +75.degree. to -75.degree.) on transverse axes so that the opposed ends of the tool slide along the engaged wires and prestresses the wires into the desired form. Repetition of this for other wire pairs defines a straight wire basket having an even number of wires with each wire extending in a substantially single plane with respect to the basket axis and in an arc-like fashion between the distal and proximal ends of the basket. This technique thus limits a straight wire retrieval basket to an even number of wires, because it operates on opposed wires in a pair simultaneously.
Alternatively, as described by Segura et al. a hand operated forming fixture acts on individual wires and includes a plurality of upwardly extending pins. The tool bends (i.e., cold works) each wire about the pins to form them in a desired loop shape. The ends of each of a plurality of individual wires, thus formed, are then secured together to define a proximal end of the straight wire retrieval basket. Since each wire comprises a first portion extending from the proximal to a distal end of the basket and a second portion extending from the distal to proximal end, straight wire baskets formed by this method also essentially comprise an even number of wires extending from the proximal to a distal end of the straight wire basket.
In forming a helical wire basket the desired number numbers of wires also are grouped together and secured at their ends along a wire axis. The ends of the subassembly are also fixed in a forming fixture. However, in this case opposed tools usually secure the wires in a generally equiangular spaced arrangement radially extended from the axis. Turning the tools about the axis in opposed senses (i.e., one is turned clockwise and the other is turned counter-clockwise) bends and stresses the wires to provide their helical, radially extending path between the ends of the basket.
These processes for forming retrieval baskets thus generally comprise the selection of wires, formation of a wire sub-assembly, and the cold working of the wires. Each method is labor intensive, as the forming fixtures of the prior art are hand operated. Significant tooling expenses are incurred as each basket size and shape generally requires its own separate tools. Further, differences between individual tools and fixtures and the processes used by individual operators leads to variations between wires of a basket and between baskets produced thereby. That is, the baskets tend to be inconsistent in their properties and characteristics. Additionally, cold-working introduces significant stress in the wires that can affect their characteristics such that individual wires have different characteristics even within one retrieval basket. Such inconsistencies often cause additional inconvenience in the use of retrieval baskets and can result in additional trauma to a patient. For example, inconsistencies in the spacing of wires can render capture and retention of objects more difficult while over stressed and/or fatigued wires can lead to premature wire failure and more fragile baskets. Frequently upon the failure of a wire in a retrieval basket, the wires tend to curl up into a ball like arrangement. The lose ends of the broken wire can injure body tissue and the ball formed by the curling up of the wires may also require major surgery to enable removal of the retrieval basket from the body.
Thus, the prior art methods fail to provide adequate controls over the forming of the wires and, thus, the basket. These methods generally do not include heat treating the wires once formed, because such heat treatment would tend to destroy the securement of the wires at the distal and proximal ends of the retrieval basket.